The invention relates to nuclear medicine, and more particularly relates to nuclear medicine SPECT studies. In its most immediate sense, the invention relates to SPECT studies that are carried out using parallel hole collimators.
In nuclear medicine studies, a radioisotope is administered to a patient and taken up in an organ of interest (e.g. the heart). The radioisotope then undergoes radioactive decay, giving off gamma radiation in the process. In Single Photon Emission Computed Tomography (SPECT) studies, a collimated detector rotates at least 180.degree. around the patient's body axis and detects the emitted gamma radiation. From this, it is possible to "reconstruct" a three-dimensional image that shows the locations within e.g. the heart where the radioisotope is taken up.
All SPECT reconstruction algorithms are based on an assumption that the coordinates of the scintillation camera system (as determined by the electronics within one or more camera detectors) correspond well to the physical coordinates of the patient. If such a correspondence is imprecise, the spatial resolution of the reconstructed SPECT images will be degraded; if such a correspondence is lacking, ring-shaped distortions (ring "artifacts") will appear in the reconstructed image.
Where the SPECT study is carried out using a focussing collimator (e.g. a collimator of the fan beam, cone beam or astigmatic type) other parameters must as well be made to correspond to the physical position of the patient. These parameters are, e.g., the radius of rotation of the detector, the x and y coordinates of the principal ray of the collimator and the focal length of the collimator.
One way to keep the coordinates of the scintillation camera system in close correspondence with the physical coordinates of the patient is to empirically determine the center of rotation ("COR") of the camera and to physically arrange the patient so that the necessary correspondence is achieved. Although this produces satisfactory results, it is inconvenient because the COR of the camera does not remain in a fixed location. COR errors arise from many sources (e.g. drift in the gains and offsets of the photodetectors in the scintillation camera detector(s), changes in camera collimation, flexure of the camera gantry) and change with time. It is therefore necessary to periodically (advantageously, monthly) redetermine the COR for each collimator that is used with the camera, using a special SPECT recalibration routine and a line of radioactive point sources.